1. Field of the Invention
The present disclosure relates to a method and device for evaluating waveforms, including acoustic waveforms. More specifically, the present disclosure relates to a method and a device for evaluating acoustic waves transmitted through a subterranean formation and received by multiple receivers.
2. Description of Related Art
Information concerning the characteristics of subterranean formations can be obtained by investigating acoustic waves that have propagated through at least a portion of the formation. Typically the investigation involves emitting one or more types of wave into the formation at one location, recording the wave at another location after it has passed through the formation, and analyzing how the wave has been affected by its travel through the formation. One common device for this investigation technique is a sonde 10 disposed in a wellbore 5 for transmitting and receiving acoustic signals. As shown, the sonde 10 is tethered to a wireline 9, control commands are provided to the sonde 10 via the wireline 9 and data recorded by the sonde 10 may be transmitted back through the wireline 9 to a surface truck 2. The sonde 10 is shown having an acoustic transmitter T1 for creating and transmitting the acoustic signals into the formation. Also included with the sonde are multiple receivers (R1-RM) disposed along the length of the sonde for receiving the acoustic signals as they have passed through the formation.
FIG. 2 provides an example of acoustic data 12 sampled by the sonde of FIG. 1. The acoustic data 12 comprises waveforms that represent acoustic signals (A1-AM) received by the respective receivers (R1-RM). Each waveform has a noise portion (N1-NM) that represents ambient noise signals recorded by each receiver and a signal portion (S1-SM) that represents the transmitted signal from the transmitter as received by the receivers. The point on the waveform at the beginning of the signal portion is typically referred to as the “first break” or “first arrival” of the acoustic signal. The moveout or slowness of the waveforms can be determined by creating a line 14 that intersects the first break of each waveform and taking the slope of that line 14.
Identifying the first break of a signal can be difficult since the magnitude of the ambient noise often equals or exceeds that of the signal itself. One technique for identifying this break point relies on the assumption that the acoustic signal received by each receiver (R1-RM) will largely have the same form. The technique involves comparing portions of the waveform of the signals (A1-AM), the initial point at which these forms largely match is determined to be the first break. As noted above however, ambient noise or noise from a monitoring device can be received by the receivers and mistaken for the actual signal—this is often referred to as a “false signal” or “false” first break detection. Thus due to the potential for detecting false signals; improved techniques for first break identification are still desired.